Support for automatic installation and configuration via environment variables

[martin-helmich]

Currently, this image is designed to drop the user into the TYPO3 installer, for them to complete the installation by themselves. However, for a cloud-native deployment, it would be much nicer for the TYPO3 image to setup "itself" and drop the user into a turn-key, freshly installed TYPO3 instance.

Roughly, this might entail the following:

• Accept a set of environment variables (to be defined) which are evaluated automatically on startup (in some kind of entrypoint script) when the TYPO3 setup had not already been completed.
• For a fully automated installation, we might need to install the TYPO3 console extension automatically. To complicate matters, different version of the TYPO3 console might be compatible with different versions of TYPO3.
• Also, we might want to run upgrade wizards on start as necessary, in case a user tries to upgrade a TYPO3 installation by simply replacing one image with a newer version

Caveats:

• I'd very much like to preserve backwards-compatibility for this image; so any added behaviour should be "opt-in", maybe only when certain environment variables are set.

Obviously, this is not entirely thought through, any may still need some love. I'll tag this as help wanted and would welcome any kind of feedback on this.